ABSTRACT
Before continuing on to the heart of our basic fluid solver, the pressure projection step to make the fluid incompressible and satisfy its boundary conditions, we need to take a diversion into geometry. In the previous chapter we already ran up against boundary conditions and related geometric problems:
when is a point inside a solid? (the point may be where we traced back to during semi-Lagrangian)
what is the closest point on the surface of some geometry?
how do we extrapolate values from one region into another?
